docs: def get_extra_radiation(doy, solar_constant=1366.1):
      """
      Determine extraterrestrial radiation from day of year (using the spencer method).
  
      Note:
          [1] M. Reno, C. Hansen, and J. Stein, "Global Horizontal Irradiance
          Clear Sky Models: Implementation and Analysis", Sandia National
          Laboratories, SAND2012-2389, 2012.
  
          [2] <http://solardat.uoregon.edu/SolarRadiationBasics.html>, Eqs.
          SR1 and SR2
  
      Args:
          doy : array of integers representing the days of the year.
          solar_constant : float, default 1366.1
              The solar constant.
  
      Returns:
          dni_extra : float, array, or Series
              The extraterrestrial radiation present in watts per square meter
              on a surface which is normal to the sun. Pandas Timestamp and
              DatetimeIndex inputs will yield a Pandas TimeSeries. All other
              inputs will yield a float or an array of floats.
      """
      # Calculates the day angle for the Earth's orbit around the Sun.
      B = (2. * math.pi / 365.) * (doy - 1)
      # Calculate R over R squared from the angle
      RoverR0sqrd = (1.00011 + 0.034221 * math.cos(B) + 0.00128 * math.sin(B) +
                     0.000719 * math.cos(2 * B) + 7.7e-05 * math.sin(2 * B))
  
      Ea = solar_constant * RoverR0sqrd
  
      return Ea